1. Field of the Invention
The present invention relates to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a communication system, an more particularly to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a telecommunication system and in a broadcasting system by use of a satellite.
2. Description of the Related Art
A telecommunication system that performs digital communication for mobile terminal devices and a broadcasting system that broadcasts programs for digital television by use of a relay station located at a high altitude such as a communication satellite or a balloon in the stratosphere have been suggested recently.
FIG. 1 is a block diagram showing a radio communication system using a communication satellite for digital communication between a base station and a radio terminal device. The radio communication system shown in FIG. 1 includes a base station 11, a relay station 12, a radio terminal device 13, and data-relay apparatuses 14 and 15.
The base station initially 11 modulates communication information, and transmits a code-division-multiplexed signal having a frequency f1 to the relay station 12 such as a communication satellite. The relay station 12 coverts the signal having the frequency f1 received from the base station 11 to a signal having a frequency f2 and a signal having a frequency f3, and transmits those signals in a code-division-multiplex format to earth stations.
The radio terminal device 13 receives the signal having the frequency f3 from the relay station 12, and demodulates the communication information included therein. The data-relay apparatus 14 receives the signal having the frequency f2 from the relay station 12, and converts the received signal to a code-division-multiplexed signal having the frequency f3, then transmitting the converted signal to other devices such as the radio terminal device 13. The data-relay apparatus 14 amplifies the received signal if necessary. The data-relay apparatus 15 receives the signal having the frequency f3 from the relay station 12, and amplifies the received signal in addition to attending to other signal processing, followed by transmitting the processed signal to other devices without converting the frequency of the received signal.
Accordingly, the radio terminal device 13 can receive the signal having the frequency f3 from the relay station 12 through the apparatuses 14 and 15 even if the radio terminal device 13 is located in a blind zone. The above-described system may also be applied to digital-television broadcasting. In this case, the system substitutes a broadcasting station for the base station 11. Additionally, a broadcasting satellite is used as the relay station 12.
FIG. 2 is a block diagram showing a conventional data-relay apparatus 14 that converts frequency of a signal to another frequency, and transmits the signal to other devices. It should be noted that the above-described conventional data-relay apparatus 14 is, hereinafter, referred to as a conventional frequency-converting-data-relay apparatus 14.
The conventional frequency-converting-data-relay apparatus 14 includes a data-receiving antenna 21, an amplifier 22, a mixer 23, a generator 24, a band-pass filter 25, an amplifier 26 and a data-transmitting antenna 27. In FIG. 2, frequency f2xe2x80x2 is the frequency f2 with an error xcex94f2. Similarly, frequency f3xe2x80x2 is the frequency f3 with an error xcex94f3.
The conventional frequency-converting-data-relay apparatus 14 initially receives a signal having the frequency f2xe2x80x2 from the data-receiving antenna 21, and amplifies the signal by use of the amplifier 22. Subsequently, the conventional frequency-converting-data-relay apparatus 14 converts the frequency f2xe2x80x2 of the signal amplified by the amplifier 22 to the frequency f3xe2x80x2 by mixing the signal amplified by the amplifier 22 with a signal generated by the generator 24 by use of the mixer 23. The frequency f3xe2x80x2 includes the error xcex94f3 of the frequency f3 that is caused by the error xcex94f2 of the frequency f2 and performance of the generator 24.
The conventional frequency-converting-data-relay apparatus 14 then filters the frequency f3xe2x80x2 of the signal by use of the band-pass filter 25. Additionally, the conventional frequency-converting-data-relay apparatus 14 amplifies the signal by use of the amplifier 26, and then outputs the signal having the frequency f3xe2x80x2 from the data-transmitting antenna 27.
FIG. 3 is a block diagram showing a conventional data-relay apparatus 15 that transmits the signal to other devices without converting frequency of a signal to another frequency. The above-described conventional data-relay apparatus 15 includes a data-receiving antenna 31, an amplifier 32, a band-pass filter 33, an amplifier 34 and a data-transmitting antenna 35.
The conventional data-relay apparatus 15 initially receives a signal having the frequency f3xe2x80x2 from the data-receiving antenna 31, and amplifies the signal by use of the amplifier 32. The conventional data-relay apparatus 15 then filters the frequency f3xe2x80x2 of the signal by use of the band-pass filter 33. Additionally, The conventional data-relay apparatus 15 amplifies the signal by use of the amplifier 34, and then outputs the signal having the frequency f3xe2x80x2 from the data-transmitting antenna 35.
In the communication system described with reference to FIG. 1, frequency deviation occurs in a signal received by the radio terminal device 13 according to performance of generators provided in the base station 11, the relay station 12 such as a satellite, and the data-relay apparatuses 14 and 15. Additionally, the frequency of the signal received by the radio terminal device 13 is shifted by the Doppler shift that is caused by movement of the radio terminal device 13 and speed of the satellite relative to the earth.
Additionally, when the radio terminal device 13 simultaneously receives a code-division-multiplexed signal having the frequency f3 from the relay station 12 and the signal having the frequency f3xe2x80x2 from the data-relay apparatuses 14 and 15, the radio terminal device 13 cannot receive the signal effectively if the frequency deviation of the signals outputted from the data-relay apparatuses 14 and 15 is large. Consequently, quality of the signal received by the radio terminal device 13 decreases.
In order to solve the above-described problems, the radio terminal device 13 may include functions to search frequency of the received signal and execute an automatic frequency control (AFC) on the frequency, thereby reducing the frequency deviation of the received signal before demodulating the received signal. However, the number of the radio terminal devices 13 in the above-described communication system is very large, especially in the broadcasting system. Thus, it is necessary to minimize production cost of the radio terminal device 13. If the radio terminal device 13 is to execute a frequency search and the AFC, a circuit arrangement of the device 13 becomes complex, and the circuit scale becomes large. As a result, the production cost of the radio terminal device 13 increases.
Accordingly, it is a general object of the present invention to provide a method and an apparatus for stabilizing frequency of an output signal of a data-relay apparatus by minimizing frequency deviation of the output signal, thereby minimizing the size of a radio terminal device that receives the output signal from the data-relay apparatus.
The above-described objects of the present invention is achieved by a method of stabilizing frequency of an output signal of a data-relay apparatus provided in a radio communication system that includes a relay station and a radio terminal device, wherein the data-relay apparatus receives a signal as an input signal from the relay station, and transmits the signal as an output signal to the radio terminal device, the method including the steps of demodulating the output signal by use of a first signal to generate a demodulated signal, detecting frequency deviation of the output signal from the demodulated signal, generating the first signal according to the frequency deviation of the output signal, and generating a second signal to mix with the input signal to generate the output signal, the generation of the first signal and the second signal being carried out such that the detected frequency deviation of the output signal becomes substantially zero.
Accordingly, the data-relay apparatus stabilizes frequency of a signal inputted thereto, and transmits the signal having the stabilized frequency to the radio terminal device in the communication system, and thus the radio terminal device does not need to execute a frequency search and AFC control so that circuit structure of the radio terminal device can be simplified and its production cost can decrease.